1. Field of the Invention
The present invention relates to a thermal analyzer, and more particularly, to heat insulation structure inside a furnace of a thermal analyzer.
2. Description of the Related Art
As an example of the thermal analyzer, a differential scanning calorimeter (hereinafter, referred to as DSC) is a thermal analyzer that changes temperature of a furnace provided inside the apparatus according to a constant temperature rate program, to thereby measure a difference in temperature between a sample and a reference substance placed inside the furnace (heat flux type, which is one type of DSCs), or a difference in thermal energy, which is applied so as to eliminate the difference in temperature between the sample and the reference substance (power compensation type, which is another type of DSCs).
In order that the DSC stably detect the difference in temperature between the sample and the reference substance or the difference in thermal energy necessary to maintain the difference in temperature therebetween to zero, it is important that a detector and a furnace portion having the detector mounted thereon are provided in a stable environment in which no direct influence of temperature disturbance is imposed. Further, from a viewpoint of providing a measurer with the convenience of being able to conduct a measurement in a wide temperature range, in order to realize a wide measurement temperature range from a desired high temperature to a temperature lower than room temperature (for example, −150° C. to 750° C.), it is also important that heat exchange between the furnace portion and the outside is suppressed to perform heating and cooling efficiently.
For the reasons described above, general DSCs are designed so that the detector and the furnace portion having the detector mounted thereon are isolated from the external environment and insulated from heat.
For example, there is proposed a heat flux DSC structured so that the entire furnace is covered with a partition wall and is further covered with a heat insulation case in which a heat insulation material is loaded into a space between an outer frame and an inner frame. The heat insulation case has an effect of suppressing influence of external temperature disturbance to provide a stable baseline, resulting in a high-sensitivity DSC measurement (see Japanese Patent Application Laid-open No. 2005-345333).
Further, for example, the power compensation DSC is structured so that temperature control can be performed both on a furnace provided with a heater for applying thermal energy to a sample and a reference substance and on a thermal shield arranged outside the furnace. By controlling temperature of the thermal shield, that is, by controlling the surrounding environment of the furnace, a stable baseline can be obtained (see Japanese Patent Translation Publication No. 2008-530560).
In the DSC measurement, sensitivity, resolution, and a noise level serve as performance indicators. In addition, baseline reproducibility is an important indicator. The “reproducibility” herein refers to “consistency of measurement baselines in repetition, which are obtained through repetitive measurements using the same temperature program”.
In a case of low (poor) baseline reproducibility, even through the repetitive measurements using the same temperature program, the baseline changes from measurement to measurement, which raises difficulty in comparing measurement results. In a case of high (good) baseline reproducibility, on the other hand, results are easy to compare between measurements, with the result that more detailed thermal changes of the sample can be captured and reliability of measurement results themselves is increased.
One of important factors of influence on the baseline reproducibility is a temperature environment given around the furnace, as well as accuracy of temperature control for the furnace that houses a detection portion. Even in a case where accurate temperature control is performed on the furnace, if the temperature environment given around the furnace fluctuates from measurement to measurement, the fluctuation of the temperature environment inevitably influences the baseline reproducibility as a measurement-based fluctuation in baseline, particularly for the high-sensitivity DSC that measures temperature or thermal energy.
However, in a thermal analyzer described in the embodiment of Japanese Patent Application Laid-open No. 2005-345333, a metal heat insulation shield and a heat insulation cover in which a heat insulation material is loaded are provided for the purpose of isolation and heat insulation of the furnace and its surroundings. In this embodiment, when repetitive measurements involving heating and cooling of the furnace are performed according to a constant temperature program, the heat insulation shield and the heat insulation cover, which are arranged around the furnace, are also heated and cooled due to the influence thereof, and temperature changes thereof occur with a delay having a fixed time constant. This is because the overall heat insulation structure around the furnace, including the heat insulation shield and the heat insulation cover, has low thermal conductivity for suppressing disturbance and a predetermined heat capacity due to the structure itself. For example, when the furnace control is switched from heating to cooling, the heat insulation material around the furnace is not so cooled as compared to the furnace itself, resulting in a delay in temperature drop. Therefore, actual temperature inside the furnace exhibits thermal hysteresis due to repetitive heating and cooling.
Due to the thermal hysteresis of the heat insulation structure, the temperature environment around the furnace is changed in the repetitive measurements, and as a result, there arises a problem of fluctuation in baseline.
In the case of the technology described in Japanese Patent Translation Publication No. 2008-530560, a thermal shield whose temperature can be controlled is provided around the furnace. It is considered that by controlling temperature of the thermal shield as appropriate, the measurement-based change of the temperature environment around the furnace does not become larger. In this case, however, it is necessary to control temperature of the thermal shield according to the state of the furnace, and hence there arises a problem of more complicated apparatus structure and control system as compared to the general case. Further, it is necessary to allocate cooling performance of a cooling device also to the thermal shield in addition to the furnace whose temperature is originally desired to be controlled, and hence the cooling rate, the lowest reachable temperature, and the like are limited as compared to the case where only the furnace is simply cooled.